The present invention relates to polymeric materials for insulating electrical equipment and providing resistance to tracking, and more particularly to such materials comprising silane-modified, moisture crosslinkable resins which are suitable for medium to high voltage insulation applications, including heat-shrinkable articles used therefor.
Polymeric materials based on polyolefins are commonly used for insulating electrical equipment since they have excellent electrical properties such as low dielectric constant and high dielectric strength, coupled with relatively low cost. Due to their semi-crystalline structure, polyolefins also exhibit good mechanical properties and can be readily crosslinked, and are therefore suitable for the manufacture of heat-shrinkable components for use in electrical insulation applications, such as heat-recoverable tubing, sheet, tape or mouldings designed to cover medium and high voltage cable splices, joints, connectors, terminations and bus-bars.
However, the medium and high voltage insulating properties of polymeric materials can be severely compromised in contaminated atmospheric conditions where deposited moisture, together with salts, dust particles, ionic pollution, acid gases and/or ultra-violet radiation reduce the surface resistivity of the insulation, thereby encouraging high leakage currents to flow across the surface of the insulation. These leakage currents may cause a rise in temperature of the polymeric material, causing surface moisture evaporation and the formation of dry solid bands of deposited material on the polymer surface. Electrical discharges or arcing can occur across these bands resulting in the degradation of the insulation and the formation of conductive carbonaceous paths on the surface. Complete failure of the system will occur when these paths propagate to the extent that the remaining insulation can no longer withstand the applied system voltage. This process is known as xe2x80x9ctrackingxe2x80x9d.
The formation of permanent conductive paths on the surface of the polymeric insulating materials has led to the development of so called xe2x80x9canti-trackxe2x80x9d polymeric compounds formulated to resist the process described above. For example, it is known that polymeric insulation materials can be rendered highly resistant to tracking by the use of certain particulate fillers, the most commonly used additives being hydrated metal oxides, such as alumina trihydrate. Other known anti-tracking fillers include magnesium hydroxide and aluminum silicate. Also, certain polymers, such as silicone elastomers are also known to impart anti-track properties.
In addition, it is beneficial to incorporate hydrophobic additives that prevent xe2x80x9cwettingxe2x80x9d of the polymer surface and the attachment of moisture and other undesirable contaminants that may promote tracking. Such additives are chosen to have low surface energy and include low molecular weight silicone and fluorine-based chemicals. It is also desirable to incorporate UV absorbing chemicals and anti-oxidant stabilisers to resist long term aging and degradation of the surface of the insulation. Examples of UV absorbers include hindered amine derivatives and certain metal oxides, such as ferric oxide. Anti-oxidants typically include hindered phenols, phosphites, and dihydroquinoline-based entities. Additional additives may include process aids, such as fatty acids and polyethylene waxes, and colorants such as ferric oxide. These additives can be incorporated into the base polymer by standard methods of melt mixing and compounding, for example using a twin-screw continuous compounder or internal batch mixing device.
Crosslinking of an insulating material as formulated above is usually necessary in the wire and cable industry to impart temperature resistance, and prevent softening and deformation of the insulation at high service temperatures. For heat-shrinkable insulations, crosslinking is also used to impart heat-recovery characteristics to the material so that it may be used to cover individual wire and cable splices, joints and terminations by applying heat to the previously crosslinked and stretched material. Crosslinking may be accomplished by the use of organic peroxides or electron beam radiation. These are commonly used techniques in the wire and cable industry. but are disadvantageous in that both processes require very large capital investment. Crosslinking by moisture using silane-modified polyolefins offers significant cost advantages over the other two methods, through this process has not previously been considered for the crosslinking of anti-track materials.
Polymeric materials used for medium and high voltage applications may also be degraded by the process known as xe2x80x9ctreeingxe2x80x9d. Not to be confused with tracking, which is a surface phenomenon, treeing is the formation and propagation of micro voids within the material due to partial discharges caused by the presence of internal moisture and impurities, coupled with electrical stress. These internal voids may gradually grow to such an extent that they ultimately reduce the integrity of the solid insulation sufficient to cause premature dielectric breakdown of the system. The cause and propagation of xe2x80x9ctreesxe2x80x9d is a distinct phenomenon and unrelated to surface tracking.
The present invention overcomes at least some of the above-discussed problems of the prior art by providing a tracking-resistant, electrical insulating material suitable for high voltage applications comprising silane-modified, moisture-crosslinkable polyolefin materials.
The silane-modified polyolefin may be prepared either by a process of grafting a vinyl silane onto an olefin homopolymer or copolymer as is described in U.S. Pat. No. 3,646,155, or, alternatively, by copolymerising the vinyl silane directly with the polyolefin as is described in U.S. Pat. No. 4,413,066, for example.
The inventors have surprisingly found that silane-modified, moisture-crosslinkable polymers, in particular, silane-grafted, moisture-crosslinkable polyolefins possess high resistance to tracking, even in the absence of conventional anti-tracking fillers such as alumina trihydrate, or other additives known to impart antitrack properties.
In the present invention, one or more silane-modified polyolefins, preferably silane-modified polyethylenes or copolymers of polyethylene, are optionally blended with one or more non silane-modified polyolefins, and other suitable additives, to produce a tracking-resistant material.
Suitable polyolefins in this invention would include those materials known in the industry as low density polyethylene, high density polyethylene, linear low density polyethylene; copolymers of polyethylene, including those based on ethylene-butene, ethylene-hexene, ethylene-octene, ethylene-vinyl-acetate, ethylene-methyl-acrylate, ethylene-ethyl-acrylate, ethylene-butyl-acrylate, and similar materials; and ethylene-propylene or ethylene-propylene diene elastomers; and, in particular those of the above prepared using so-called metallocene catalysts. Additionally, the non silane-modified polymer may not necessarily be restricted to polyolefins, but may include other suitable polymers, such as silicone elastomers and fluoropolymers which are known to resist tracking.
The blended composition is then formed into the desired article by melt processing techniques, such as extrusion and moulding. The article thus formed is then crosslinked in the presence of a silanol condensation catalyst under suitable conditions of moisture, preferably in the presence of heat, the catalyst being either blended with the composition during melt processing or added subsequent to melt processing by surface coating the article.
Accordingly, the present invention provides an electrical insulating material suitable for high voltage applications which is resistant to tracking and spark erosion, comprising a silane-modified, preferably silane-grafted, moisture-crosslinked polymeric material formed by a process comprising: (a) reacting polyolefin with a silane to form a silane-grafted resin or silane-olefin copolymer; (b) producing a mixture of the silane-modified polyolefin, and optionally one or more non silane-modified polyolefins and/or suitable additives, and a silanol condensation catalyst; (c) forming the insulating material by melt extruding the mixture formed in step (b); and (c) crosslinking the insulating material by exposing it to moisture, preferably at elevated temperature.